MEMS device having a flexure with integral actuator

ABSTRACT

A micro-electro-mechanical device comprises a moveable mass, a frame for supporting the mass, and a flexure extending between the mass and the frame. The flexure includes an integral actuator for moving the mass member with respect to the frame.

REFERENCE TO CO-PENDING APPLICATION

[0001] Reference is made to co-pending U.S. patent application Ser. No.______, entitled “FLEXURE WITH INTEGRAL ACTUATOR”, having AttorneyDocket No. 10015371-1, filed on like date herewith and having commoninventorship and assignment.

THE FIELD OF THE INVENTION

[0002] The present invention generally relates tomicro-electro-mechanical devices, and more particularly to amicro-electro-mechanical device having a moveable mass which issupported and moved by a flexure having an integrally formed actuator.

BACKGROUND OF THE INVENTION

[0003] Micro-electro-mechanical systems (hereinafter “MEMS”) areintegrated systems of small size where the feature sizes are generallyof micron dimensions. MEMS devices are created on a common siliconsubstrate utilizing microfabrication technology like that used forintegrated circuit (IC) processing. The fabrication processesselectively etch away parts of the silicon wafer or add new structurallayers to form the mechanical and electromechanical devices.

[0004] One unique feature of MEMS is the extent to which actuation,sensing, control, manipulation, and computation are integrated into thesame system. Examples of MEMS devices include individually controlledmicro-mirrors used in a projection display, accelerometers that sense acrash condition and activate airbags in cars, pressure sensors, “lab ona chip” systems, and data storage devices.

[0005] Many MEMS devices include masses that are moveable within thesystem. In these MEMS devices, beams or flexures are often used tosupport the moveable masses in the system. The beams supply both supportof the system's mass and compliance for the system's mass movements. Ifmotion of the system's mass must be limited, additional features aregenerally created in the system to limit the motion as desired. Theactual movement of a system's mass is accomplished by yet another deviceseparate from the beams or flexures and motion limiting features.Referred to herein generically as actuators or microactuators, varioustypes of devices may be used to cause movement of a system's mass.Micro-actuators which are used in MEMS devices use a variety of methodsto achieve actuation: electrostatic, magnetic, piezoelectric, hydraulicand thermal.

[0006] In MEMS devices such as those mentioned above, space limitationsof the device must be considered. Even though MEMS devices are bydefinition already extremely small, it may be desired to maximize thesize of one component of the device relative to the size of anothercomponent or to the size of the entire device. Thus, it would bedesirable to reduce the space occupied by such other components of thedevice, or to eliminate selected components entirely. In addition, itwould be desirable to reduce the number of process steps needed tofabricate a particular MEMS device or specific components of a MEMSdevice. As noted above, MEMS devices are manufactured usingmicrofabrication technology like that used in the production ofintegrated circuits. A reduction or simplification of the process stepsrequired to form a particular MEMS device or one of its components wouldspeed the manufacturing process, and reduce the likelihood of error inthe manufacturing process.

[0007] In the example of MEMS devices having masses that are moveablewithin the system, it is often the moveable mass whose size is desiredto be maximized with respect to the total size of the device. Forexample, in a data storage device, the moveable mass may be or includethe storage medium. To maximize the data storage capacity of the device,it would be desirable to make the moveable mass as large as possiblewithin the confines of the device. In such devices, it would bedesirable to reduce the total space occupied by the flexures supportingthe moveable mass, the features limiting the mass motion, and theactuator that moves the mass.

SUMMARY OF THE INVENTION

[0008] A micro-electro-mechanical device comprises a moveable mass, aframe for supporting the mass, and a flexure extending between the massand the frame. The flexure includes an integral actuator for moving themass with respect to the frame.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009]FIG. 1 is a perspective view of one embodiment of a flexure usedin one embodiment of the invention.

[0010]FIG. 2 is a top plan view of the flexure shown in FIG. 1.

[0011]FIG. 3 is a greatly enlarged view of the circled portion 3 of FIG.1.

[0012]FIGS. 4a and 4 b are top plan views of another embodiment of theflexure used in another embodiment of the invention.

[0013]FIGS. 5a and 5 b are top plan views of additional embodiments ofthe flexure used in additional embodiments of the invention.

[0014]FIGS. 6a and 6 b are a plan view and a perspective view,respectively, of the one embodiment of the inventive MEMS device using aflexure having an integral actuator.

[0015]FIGS. 7a and 7 b are alternate embodiments of one portion of theMEMS device of FIGS. 6a and 6 b.

[0016]FIG. 8 is an illustration of beam movement and torsion in the MEMSdevice of FIGS. 6a and 6 b.

[0017]FIG. 9 is a plan view of another embodiment of a MEMS device usinga flexure having an integral actuator.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] In the following detailed description of the preferredembodiments, reference is made to the accompanying drawings which form apart hereof, and in which is shown by way of illustration specificembodiments in which the invention may be practiced. It is to beunderstood that other embodiments may be utilized and structural orlogical changes may be made without departing from the scope of thepresent invention. The following detailed description, therefore, is notto be taken in a limiting sense, and the scope of the present inventionis defined by the appended claims.

[0019] One embodiment of a flexure 10 for use in amicro-electro-mechanical system (MEMS) is shown in FIGS. 1-3. Theflexure 10 includes a compliant longitudinal beam 12 having a first side14 and a second side 16 opposite first side 14. An actuator 18 isintegrally formed with beam 12. The actuator 18 may be selectivelyactivated to flex beam 12. In use, flexure 10 may, for example, bepositioned to interact with a movable mass (not shown), such thatflexing of beam 12 by actuator 18 moves the moveable mass.

[0020] As shown in FIGS. 1 and 2, actuator 18 is integrally formed aspart of a central section 20 of beam 12. However, actuator 18 may bepositioned along any portion of the length of beam 12 (such as adjacentan end 23 of beam 12), or along the entire length of beam 12, as isrequired for a particular application. The end sections 21 of beam 12may be of any length appropriate for the flexures intended use. Thedimensions of the embodiment of invention shown in FIGS. 1-3 should notbe construed as limiting with regard to the dimensions and positioningof actuator 18.

[0021] In the embodiment of flexure 10 shown in FIGS. 1-3, actuator 18is of an electrostatic nature. That is, actuator 18 is selectivelyactivated by the presence of an electrostatic charge. In one embodimentof the invention, actuator 18 comprises a plurality of force elements 22projecting from the first side 14 of beam 12. When flexure 10 iselectrostatically charged, force elements 22 move apart from each otherdue to a repulsive electrostatic force and thereby flex beam 12 towardsits second side 16.

[0022] Force elements 22 may also be positioned on both sides of beam12. As shown in FIG. 4a, force elements 22 are positioned adjacent bothfirst side 14 and second side 16 of beam 12. The group of force elements22 positioned on first side 14 in the central section 20 of beam 12 willact to flex end sections 21 of beam 12 toward second side 16. At thesame time, the group of force elements 22 positioned on second side 16in the end sections 21 of beam 12 will act to flex the ends 23 of beam12 back toward first side 14. When electrostatically charged, beam 12will have a shape similar to that shown in FIG. 4b (the shape of beam 12in FIG. 4b is greatly exaggerated for illustrative purposes). Forceelements 22 may be positioned along the beam 12 in configurations otherthan that shown which result in the desired shape of beam 12 whenelectrostatically charged.

[0023] Force elements 22 also function to limit the bending or flexingof beam 12 toward the side of the beam 12 with force elements 22. In theembodiments shown in FIGS. 1-3, force elements 22 limit the bending ofbeam 12 toward its first side 14. In particular, as beam 12 flexestoward its first side 14, force elements 22 contact each other andthereby prevent further flexing or bending of beam 12 in that direction.In this manner, additional elements intended to limit the movement ofbeam 12 or a mass with which it interacts do not need to be incorporatedin the MEMS device using flexure 10.

[0024] When used as a micro-electro-mechanical device, beam 12 may havea width between the first side 14 and second side 16 in the range of100,000 angstroms (10 microns) or less, and more typically less than30,000 angstroms (3 microns), depending upon the intended application offlexure 10. Beam 12 may also preferably be a high aspect ratio beam. Inone possible embodiment, beam 12 will have an aspect ratio of at least3, but the aspect ratio may be much more or less depending upon theapplication. A high aspect ratio in beam 12 creates more surface areabetween adjacent force elements 22, and thus creates a larger actuationforce between adjacent force elements 22 when flexure 10 iselectrostatically charged. The aspect ratio of beam 12 will beinfluenced by factors including the force required to be generated byactuator 18 of flexure 10, the strength of the electrostatic charge andthe amount of available space in the MEMS device.

[0025] In one embodiment of the invention, force elements 22 compriseT-shaped (or hammer-shaped) elements which are monolithically attachedto and extend from the first side 14 of beam 12. Each T-shaped elementcomprises a T-stem 24 and a T-cross member 26, with the T-stem 24extending from beam 12. T-cross members 26 move apart from each otherwhen electrostatically charged to flex beam 12 toward second side 16.T-cross members 26 will contact each other if beam 12 is flexed towardfirst side 14, and thereby limit the degree to which beam 12 can flextoward first side 14.

[0026] Force elements 22 may have shapes other than a T-shape asillustrated in FIGS. 1-3. For example, force elements 22 may be straight(FIG. 5a), L-shaped (FIG. 5b), or any other shape which may used tocreate a repulsive force when electrostatically charged.

[0027] One possible application for the flexure 10 having an integrallyformed actuator 18 as described above is illustrated in FIGS. 6a and 6b. FIGS. 6a and 6 b show a MEMS device having a moveable mass. Manytypes of MEMS devices utilize moveable masses, but for ease ofexplanation a high-density data storage module 110 is described herein.However, it is intended that the invention be defined by the language ofthe claims below and not restricted to data storage modules. Storagemodule 110 includes a rotor 112 and a frame 114 for supporting rotor112. Rotor 112 is bounded by its top edge 116, bottom edge 118, leftedge 120 and right edge 122. The front face 124 of rotor 112 defines anX-Y plane, with top edge 116 and bottom edge 118 aligned with theX-axis, and left edge 120 and right edge 122 aligned with the Y-axis.Front face 124 of rotor 112 is formed from a storage medium that has aplurality of storage areas 126 for data storage. The storage areas 126(shown generically in FIG. 6b) are in one of a plurality of states torepresent data stored in that area. Rotor frame 114 is spaced from rotoredges 116, 118, 120 and 122. In one embodiment, rotor frame 114surrounds rotor 112 in the X-Y plane. (As used herein, directional termssuch as top, bottom, left, right, front and back are relative terms, andshould not be construed as a limitation on the overall orientation ofthe storage module 110).

[0028] Rotor 112 is supported within rotor frame 114 by a plurality offlexures 10 which interconnect rotor 112 and rotor frame 114. Forceelements 22 of flexures 10 are not illustrated in FIGS. 6a and 6 b forreasons of clarity. However, flexures 10 are of the type described abovehaving integrally formed actuators 18. The flexures 10 supply bothsupport of the rotor 112 and compliance for movements of rotor 112. Incontrolling the motion of rotor 112, it is often desirable to have thegreatest in-plane to out-of-plane compliance ratio (referred to hereinas the compliance ratio) possible. However, this ratio can be limited bythe chosen mechanical architecture. The reason a high compliance ratiois desirable is that the forces provided by the actuator 18 integrallyformed in flexures 10 are not very strong. Improving in-plane compliancewhile maintaining, or improving, the compliance ratio allows therelatively weak forces of integral actuators 18 to move rotor 112 in anacceptable manner. Increasing the in-plane compliance may beaccomplished by allowing for axial shortening of the flexures 10. Thatis, as the flexures 10 bend they tend to become shorter in their axialdirection which leads to a decrease in the in-plane compliance.Compensating for this axial shortening will increase the in-planecompliance. An additional way to improve the in-plane compliance whilekeeping the out-of-plane compliance low and still improving thecompliance ratio is to allow the ends of the flexures 10 to moveangularly. Even a small angle at either or both ends of the beam 12 cansignificantly increase the in-plane compliance. In many instances, thesame structure may compensate for axial shortening and also allowangular movement of the beam.

[0029] As shown in FIGS. 6a and 6 b, to compensate for axial shorteningand also allow angular movement of the flexures 10, a first pair ofcoupling beams 130 a, 130 b extend from top edge 116 of the rotor 112,while a second pair of coupling beams 132 a, 132 b extend from bottomedge 118 of rotor 112. In the embodiment shown in FIGS. 6a and 6 b,rotor 112 is rectangular in shape and first set of coupling beams 130 a,130 b, 132 a, 132 b extend from the corners of rotor 112. Coupling beams130 a, 130 b, 132 a, 132 b are generally aligned with the left and rightedges 120, 122 of rotor 112. However, coupling beams 130 a, 130 b, 132a, 132 b may have a different origination and orientation from thatshown in FIGS. 6a and 6 b. For example, the alternate embodiments shownin FIGS. 7a and 7 b allow coupling beam 130 a additional freedom torotate and thereby provide additional in-plane compliance to the rotor112.

[0030] First pair of coupling beams 30 a, 30 b are connected to firstcoupling mass 134 a (positioned adjacent top edge 116 of rotor 112) byflexures 136 a extending between the first pair of coupling beams 130 a,130 b and first coupling mass 134 a. Second pair of coupling beams 132a, 132 b are connected to second coupling mass 134 b (positionedadjacent bottom edge 118 of rotor 112) by flexures 136 b extendingbetween the second pair of coupling beams 132 a, 132 b and secondcoupling mass 134 b. First set of flexures 136 a, 136 b, have an axialorientation which is generally aligned with the top and bottom edges116, 118 of rotor 112.

[0031] Rotor frame 114 includes first and second flexure mounts 140 a,140 b, which are positioned on opposite sides of rotor 112 (adjacentleft edge 120 and right edge 122 as shown in FIG. 6a). First and secondcoupling masses 134 a, 134 b are connected to first flexure mount 140 aby flexures 142 a. First and second coupling masses 134 a, 134 b areconnected to second flexure mount 140 b by flexures 142 b. Second set offlexures 142 a, 142 b have an axial orientation which is generallyaligned with the left and right edges 120, 122 of rotor 112. Couplingmasses 134 a, 134 b simply act as rigid bodies to translate movementbetween flexures 142 a, 142 b and flexures 136 a, 136 b.

[0032] It should be noted that in the embodiment shown in FIGS. 6a and 6b, the sets of flexures 136 a, 136 b, 142 a, 142 b each comprise a totalof four individual flexures. However, a different number of individualflexures may be used in the sets of flexures (for example, a total oftwo or six flexures in each set).

[0033] The faces of flexures 136 a, 136 b are in the X-Z plane; this setof flexures may be flexed in the Y direction allowing the rotor 112 tomove in the Y direction with respect to the frame 114. The faces offlexures 142 a, 142 b are in the Y-Z direction; this set of flexures maybe flexed in the X direction allowing the rotor 112 to move in the Xdirection with respect to the frame 114.

[0034] A simplified axial view of one of the high aspect beam flexures10 is shown in FIG. 8. As the beams 12 are flexed in-plane andout-of-plane, a torsion occurs in the beams 12. This torsion occurs eventhough the beam 12 does not twist with respect to its axial plane. FIG.8 shows end views of a high aspect ratio beam under no load (PositionA), in-plane and out-of-plane loads (Position B), and in-plane,out-of-plane and torsion loads (Position C). Because the motion of therotor 112 puts the beam 12 in torsion due to the moment arms arisingfrom displacement, the beam's tendency is to flex back from the PositionC illustrated in FIG. 8 toward the Position B illustrated in FIG. 8. Asnoted above, it is often desirable to have the greatest in-plane toout-of-plane compliance ratio possible. However, this compliance ratiois often decreased by the beam torsions described above. In order tomaintain a higher compliance ratio, it is desirable to decrease thebeam's torsional and out-of-plane compliance while maximizing itsin-plane compliance.

[0035] In the high density storage module described herein, the beamstorsional and out-of-plane compliance is reduced by aligning theflexures 10 in such a way as to effectively counteract the torsionscreated in the flexures 10 as the rotor 112 is displaced along theZ-axis, such as by vibrational forces. The greatest counteraction effectis achieved when flexures 136 a, 136 b are oriented to axially point atthe midpoint of flexures 142 a, 142 b. However, counteraction of thetorsions are also achieved the lesser extent when the intersection isnot at the midpoint of flexures 142 a, 142 b. Thus, the position of thefirst and second set of flexures 136 a, 136 b, is such that the axis ofthe first and second set of flexures 136 a, 136 b, intersects theflexures 142 a, 142 b somewhere along the length of flexures 142 a, 142b.

[0036] Although the storage module 110 has been described above withrespect to a single rotor 112 supported by frame 114, in practice aplurality of rotors 112 may be supported by frame 114. A storage module210 having an array of rotors 112 is illustrated in FIG. 9. It will benoted that the orientation of flexures 136 a, 136 b, 142 a, 142 bprovides a significant benefit when a plurality of rotors 112 are usedin the storage module 210. Specifically, flexures 136 a, 136 b, 142 a,142 b are arranged about the periphery of rotors 112 such that flexures136 a, 136 b, 142 a, 142 b are each in substantially parallel alignmentwith the respective adjacent edges of rotors 112. Thus, the total arearequired for each rotor 112 and its associated suspension system isreduced and the packing density of rotors 112 within storage module 210is correspondingly increased.

[0037] The packing density of the rotors 112 in storage module 210 maybe further increased, as illustrated in FIG. 9, by eliminating themajority of the frame 114 between adjacent rotors 12. Specifically, itcan be seen in FIG. 9 that the frame 114 is reduced to leave only theflexure mounts 140 a, 140 b of adjacent rotors 112. That is, the onlyportion of frame 114 between adjacent rotors is the flexure mounts 140a, 140 b. The flexure mounts are mechanically secured to a motionground, so that each rotor of the array of rotors 112 may moveindependently. Of course, frame 114 may also be extended so that itfully surrounds each rotor, if that is desired.

[0038] Although specific embodiments have been illustrated and describedherein for purposes of description of the preferred embodiment, it willbe appreciated by those of ordinary skill in the art that a wide varietyof alternate and/or equivalent implementations calculated to achieve thesame purposes may be substituted for the specific embodiments shown anddescribed without departing from the scope of the present invention.Those with skill in the chemical, mechanical, electromechanical, andelectrical arts will readily appreciate that the present invention maybe implemented in a very wide variety of embodiments. This applicationis intended to cover any adaptations or variations of the preferredembodiments discussed herein. Therefore, it is manifestly intended thatthis invention be limited only by the claims and the equivalentsthereof.

What is claimed is:
 1. A micro-electro-mechanical device comprising: amoveable mass; a frame for supporting the mass; and a first flexureextending between the mass and the frame, wherein the first flexureincludes an integral actuator for moving the mass with respect to theframe.
 2. The device of claim 1, further comprising a second flexureextending between the mass and the frame, wherein the second flexureincludes an integral actuator for moving the mass with respect to theframe.
 3. The device of claim 2, wherein the first flexure moves themass in a first direction and the second flexure moves the mass in asecond direction.
 4. The device of claim 3, wherein the first directionis opposite the second direction.
 5. The device of claim 3, wherein thefirst direction is normal to the second direction.
 6. The device ofclaim 1, wherein the integral actuator comprises a plurality of forceelements on the flexure.
 7. The device of claim 6, wherein the pluralityof force elements are moveable apart from each other at the urging of arepulsive electrostatic force to flex the flexure and move the massmember with respect to the frame.
 8. The device of claim 7, wherein eachof the plurality of force elements comprises a T-shaped elementmonolithically formed with and extending from a side of the flexure. 9.The device of claim 7, wherein the plurality of force elements arepositioned on a single side of the flexure.
 10. The device of claim 7,wherein the plurality of force elements are positioned on more than oneside of the flexure.
 11. A data storage module for a data storagedevice, the storage module comprising: a rotor having a plurality ofdata storage areas, the storage areas each being in one of a pluralityof states to represent the data stored in that area; a frame; a firstset of flexures suspending the rotor within the frame and permitting therotor to move along a first axis; and a second set of flexuressuspending the rotor within the frame and permitting the rotor to movealong a second axis; and the flexures of the first and second sets offlexures having monolithic force elements along their length for flexingthe flexures and moving the rotor with respect to the frame.
 12. Thedata storage module of claim 11, wherein the first set of flexures haveaxes that are normal to the first axis.
 13. The data storage module ofclaim 11, wherein the second set of flexures have axes that are normalto the second axis.
 14. The data storage module of claim 11, wherein theaxes of the first set of flexures intersects the second set of flexuresalong a length of the second set of flexures.
 15. The data storagemodule of claim 11, wherein the first and second sets of flexurescomprise thin-walled micro-fabricated beams.
 16. The data storage moduleof claim 11, further comprising: a first set of coupling beams extendingfrom the rotor, wherein the first set of flexures extend between thefirst set of coupling beams and a coupling mass.
 17. The data storagemodule of claim 16, wherein the second set of flexures extend betweenthe coupling mass and the frame.
 18. The data storage device of claim17, wherein the first set of coupling beams have axes that are alignedwith the first axis.
 19. The data storage module of claim 11, furthercomprising: a plurality of rotors, each being similar to the rotorrecited in claim 11, each of the plurality of rotors suspended withinthe frame by flexures similar to the flexures recited in claim
 11. 20.The data storage module of claim 11, wherein the first and second setsof flexures comprise micro-fabricated beams.